2.2. Case Sensitivity

JavaScript is a case-sensitive
language. This means that language
keywords, variables, function names, and
any other identifiers must always be typed with a consistent
capitalization of letters. The while keyword, for
example, must be typed "while", not "While"
or "WHILE". Similarly, online,
Online, OnLine, and
ONLINE are four distinct variable names.

Note, however, that HTML is not case-sensitive. Because of its close
association with client-side JavaScript, this difference can be
confusing. Many JavaScript objects and properties have the same names
as the HTML tags and attributes they represent. While these tags and
attribute names can be typed in any case in HTML, in JavaScript they
typically must be all lowercase. For example, the HTML
onclick event handler attribute is commonly
specified as onClick in HTML, but it must be
referred to as onclick in JavaScript code.

While core JavaScript is entirely and exclusively case-sensitive,
exceptions to this rule are allowed in
client-side
JavaScript. In Internet Explorer 3, for example, all client-side
objects and properties were case-insensitive. This caused problematic
incompatibilities with
Netscape, however, so in Internet
Explorer 4 and later, client-side objects and properties are
case-sensitive.